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buckling analysis under L.L. for aircraft composite structure

buckling analysis under L.L. for aircraft composite structure

buckling analysis under L.L. for aircraft composite structure

(OP)
Hello
I'm working on the sizing of the aircraft structure in composite, I'm in the phase of preliminary load.
I did a first size at Ultimate Load considering the strain check and the buckling check, all the MS>0: The post-buckling isn't allowed under Ultimate Load.
Now I'm considering the Fail Safe under Limit Load. Surely I have to do the strain check (MS>0).
What about the buckling check, do I check buckling also (as I did for Ultimate Load Cases)?

I think that the answer is in the FAR and JAR ...

May you suggest me in which FAR/JAR I can read about sizing guideline in Failure Load Case (L.L.)?

Thanks in advance.

RE: buckling analysis under L.L. for aircraft composite structure

i'd be surprised if the answer is in the FARs, just possibly it might be an AC but usually this is a company decision.  if you're not allowed post-buckling under limit load, then you've got the same riteria for limit, you're basing your allowables on elastic buckling (the onset of buckling).

RE: buckling analysis under L.L. for aircraft composite structure

This sort of criteria is not defined in the FARs/JARs/ACs.  It should/would be defined in the company/program design criteria document.

If you are designing to be stable at ultimate, I would suspect that you should design to be stable to limit for fail safe load cases.  Otherwise, you are going to need test data to validate post-buckled capability with damage.

What specific structure are you sizing, and what specific damage cases are you considering for Fail Safe.

RE: buckling analysis under L.L. for aircraft composite structure

Agree.

Post-another-part's-failure, a failsafe part checked for ultimate failure at limit load (and/or onset of buckling), usually doesn't have any further proof-criteria: you don't have to check for metallic yield and analagous composite effects at less than limit load. However, there should be clear design sizing requirements documented.
 

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